1. Field of the Invention
The present invention generally relates to dielectric filters having at least one dielectric resonator, the dielectric resonator having an internal conductor which is formed within a dielectric block and an external conductor which is formed on the outside of the dielectric block.
2. Description of Related Art
Filters for use in, for example, the microwave band, include a dielectric filter, in which a resonator electrode is formed within a dielectric block and an earth electrode is formed on the outside face of the dielectric block, and a so-called Triplate (TM) type of dielectric resonator with strip lines located opposite to each other on respective main faces of a dielectric substrate, the strip lines serving respectively as a signal strip line on one main face and an earth electrode on the other main face.
FIG. 39 shows an exploded perspective view of the construction of the conventional general dielectric resonator using a dielectric block. In FIG. 39, reference numeral 40 is a six-sided dielectric block with three internal conductor holes 46, 47, 48 each having an internal conductor provided therein and coupling holes 49, 50 which are provided between the internal conductor holes 46, 47, 48. The internal conductors are formed on the inside surfaces of the internal conductor holes 46, 47, 48, and an external conductor 51 is formed on five faces of the dielectric block 40 except for an open face 52. Reference numerals 53, 54 are so-called resin pins, each being composed of resin portions 53a, 54a and signal input, output terminals 53b, 54b. Two resin pins 53, 54 are inserted into the internal conductor holes 46, 48 from the open face side of the dielectric block 40 so that the terminals 53b, 54b are coupled capacitively to the corresponding internal conductors within the internal conductor holes 46, 48. Reference numeral 55 is a case for retaining the dielectric block 40 and the resin pins 53, 54 and also, for covering the open face portion of the dielectric block 40. The resin pins 53, 54 are respectively inserted into the dielectric block 40 so as to be covered by the case 55, and also, the whole arrangement is integrated by soldering the case 55 to the external conductor 51. For mounting the dielectric resonator on a circuit substrate, the projecting portions 55a, 55b of the case 55 function as an earth terminal.
As shown in FIG. 39, many components such as input, output terminals 53b, 54b, case 55 and so on, are necessary if a plurality of resonators are to be formed in a single dielectric block. The assembly steps therefore become complicated. Moreover, it is necessary to attach a lead wire to the component when mounting the completed product on a circuit substrate. Therefore, surface mounting cannot be effected, as it can with other electronic components, so as to mount a plurality of these completed products on the same circuit substrate. Thus, it is difficult to provide an assembly which is low in height.
Further, if the case 55 is not used, the external conductor 51 of the dielectric block 40 is directly connected to the earth electrode on the circuit substrate, so that the open face 52 is exposed, and thus, electromagnetic field leakage occurs at this location. Thus, when a metallic object approaches the open face 52, the metallic object influences this electromagnetic field. Further, since the resonator is coupled with this electromagnetic field, the desired characteristics of the dielectric resonator cannot be obtained.